libata: add private driver field to struct ata_device

This brings struct ata_device in-line with struct ata_{port,host}.
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

include/linux/libata.h

@@ -595,6 +595,7 @@ struct ata_device {
 	unsigned int		horkage;	/* List of broken features */
 	unsigned long		flags;		/* ATA_DFLAG_xxx */
 	struct scsi_device	*sdev;		/* attached SCSI device */
+	void			*private_data;
 #ifdef CONFIG_ATA_ACPI
 	acpi_handle		acpi_handle;
 	union acpi_object	*gtf_cache;